Competing quantum effects could explain the melting point of water.
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Water is a familiar substance. It is vital to life and covers over 70% of the surface of the Earth. But we’re less familiar with the quantum mechanics that controls the structure and dynamics of water on the atomic scale – including the temperatures at which water boils and melts. An international group of researchers have been using the VESUVIO instrument at ISIS to study competing quantum effects, explaining why the melting points of light and heavy water are surprisingly similar. Their research has been published in the Journal of Physical Chemistry Letters.
Quantum mechanics plays a major role in determining how light nuclei behave, which in turn affects the physical properties of materials such as heat capacity. In heavy water, the lighter hydrogen-1 isotope is replaced by the heavier deuterium isotope. As a result, one might expect quantum effects to lead to significantly difference boiling and melting temperature, but in fact this is not the case – there is less than 4K between melting points and about 1.4K between boiling points.
Recent theoretical work has proposed that the reason for this is competing nuclear quantum effects (NQEs) along different molecular axes partially cancelling each other out to reduce the net effect. The work on VESUVIO could observe this competition directly, by resolving for the first time the components of the quantum kinetic as it changes upon melting. The researchers used deep inelastic neutron scattering (DINS) to measure deuterium and oxygen momentum distributions in heavy water and ice. VESUVIO is the only instrument capable of performing measurements of the nuclei's momentum distribution using this technique. This allowed them to directly verify the existence of competing NQEs.
"Theoretical analysis and indirect experimental evidence—such as the shift of vibrational peaks in infrared spectroscopy–suggested the concept of competing quantum effects," says coauthor Michele Ceriotti at the University of Oxford. "This study provides a very direct demonstration of this concept, by measuring the change of the quantum kinetic energy of deuterium and oxygen in heavy water upon melting. A degree of competition between different components of nuclear quantum effects can be expected in most hydrogen-bonded systems, and this work demonstrates the possibility of accessing this information directly by deep inelastic neutron scattering experiments."
Roberto Senesi, from the University of Rome Tor Vergata in Rome, Italy, adds, “This is the first measurement of the oxygen momentum distribution in water, made possible by the most recent advances in Electron-volt Neutron Spectroscopy at ISIS, and our experiment gives a nice example of what VESUVIO can offer."
Research date: November 2013
Giovanni Romanelli, et al. "Direct Measurement of Competing Effects on the Kinetic Energy of Heavy Water upon Melting." The Journal of Physical Chemistry Letters, 2013, 4, 3251-3256. DOI: 10.1021/jz401538r
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